1. Field of the Invention
This invention relates to the field of current source and mirror circuits, particularly those designed for use with low supply voltages.
2. Description of the Related Art
Integrated circuit components continue to shrink in size, and demands on circuit efficiency, particularly in battery-powered devices, continue to increase. One way to accommodate these trends is to lower a circuit's supply voltage, which reduces the amount of heat which must be dissipated as well as the drain on the battery.
However, as supply voltages fall, some commonly used analog circuit building blocks no longer function as designed. One such circuit is shown in the schematic diagram of FIG. 1, which is designed to generate a complementary-to-absolute temperature (CTAT) current from a poorly controlled source of current i.sub.poor. A "poorly controlled" source of current is characterized as a current whose precise magnitude is unpredictable. Such a current may arise, for example, in a voltage regulator circuit at start up. The poorly controlled current i.sub.poor is connected to the collector of a pnp transistor QA and the base of a pnp transistor QB whose emitter is connected to the base of QA. QA's emitter is connected to a supply voltage V+ and the collector of QB serves as the output 10 of the current source. A resistor R is connected across the base and emitter of QA.
In operation, the base of QB is pulled low by i.sub.poor until QA is turned on and supplies i.sub.poor. When turned on, QA's base-emitter voltage V.sub.beA is across R. Since V.sub.beA responds only weakly to i.sub.poor, but strongly to temperature, the current through R and passed by QB to the output 10 is nominally CTAT.
The voltage drop from the emitter of QA to the base of QB is equal to the base-emitter drop of QA plus that of QB (V.sub.beA +V.sub.beB), which may equal 1.6 volts or more, particularly at cold temperatures. This works well with a V+ of 3 volts or more, but will not operate with a supply voltage of 1.5 volts or less, as modern-day circuits increasingly demand. Moreover, the output terminal cannot approach the rail voltage (V+) any closer than V.sub.satB (QB's saturation voltage)+V.sub.beA.
Current mirror designs are also challenged by the use of lower supply voltages. A Wilson mirror is widely used where performance superior to a simple current mirror is needed, and is shown in FIG. 2. Here, a programming current i.sub.pgm is sourced by a transistor QC, which is mirrored by a diode-connected transistor QD connected to have the same base-emitter voltage as QC. A cascode transistor QE is inserted between QD and the load to pin the collector of QC at two junction voltage drops below V+ and thus reduce voltage variations across QC. Use of this circuit, however, requires that V+ be equal to at least 2 junction voltage drops, or two gate voltages for an FET implementation, in series.
The differential amplifier is another analog circuit building block, an example of which is shown in the schematic diagram of FIG. 3. Transistors QF and QG form a differential amplifier, biased with a current source i.sub.diff connected to their respective emitters. A differential input voltage V.sub.in is applied across QF's and QG's respective bases and a differential output current appears at their respective collectors. The collectors of two current mirror transistors QH and QJ are connected to the collectors of QF and QG, respectively, to provide a differential to single-ended converter for the differential output current, with a single-ended output taken from QJ's collector. The differential amplifier and its tail current source typically consume about one volt of headroom at low temperatures, with another 0.6-0.9 volts lost due to the base-emitter voltages of converter transistors QH and QJ. Thus, this conventional differential amplifier and converter is also rendered unusable at supply voltages below about 1.6 volts.